Stannous fluoride reacts with tooth mineral to form Sn3F3PO4, although other products are possible under somewhate different experimental conditions: CaF2, SnHPO4, and Sn4(PO4)2(OH)2. The rate of Sn3F3PO4 is to be studied experimentally as a function of various combinations of stannous, fluoride, and hydrogen ion concentrations. Such information should be useful in the design of more efficacious topical preparations.